It is known that when the rolling bearing in which grease is enclosed is used in a high load-applied condition, a lubricating film of lubricating grease is liable to fracture. When the lubricating film fractures, metal contact occurs to generate disadvantages of defective lubrication that causes an increase in heat generation and in frictional wear. Therefore by using grease containing an extreme-pressure agent (EP agent), the disadvantages caused by the metal contact is prevented. As substances for preventing the above-described disadvantages, solid lubricant-containing grease (see patent document 1) containing 100 parts by weight of an adduct of melamine (iso) cyanurate and 5 to 1000 parts by weight of a solid lubricant selected from the group of polytetrafluoroethylene, molybdenum disulfide, and molybdenum dithiocarbamate (thereinafter abbreviated as MoDTC) is disclosed (see patent document 1). To decrease wear, a grease composition (see patent document 2) containing molybdenum dithiocarbamate and polysulfide is also disclosed.
When the grease composition is used for the rolling bearing and the constant velocity joint which are used for the rolling stock, the building machine, the steel paper-making machine, the car wheel indication part and the like under high load-applied conditions, the conventional grease composition has a problem that it has an insufficient wear-reducing effect. In severe use conditions, it is necessary to improve the grease composition in lubricating properties and properties of withstanding a high load to prevent the metal contact which occurs owing to the fracture of the lubricating film. In the roller bearing, rolling friction is generated between the rolling surface of the inner ring as well as the rolling surface of the outer ring and a “roller” which is a rolling element and sliding friction is generated between the flange portion and the “roller”. Because the degree of the sliding friction is larger than that of the rolling friction, the fracture of lubricating film is liable to occur at the flange portion.
In the ball bearing, sliding occurs between the rolling element and the cage, and further differential sliding occurs between the rolling element and the bearing ring. Thus the lubricating oil film of the lubricating grease is liable to fracture.
In using the extreme-pressure agent containing sulfur in the molecular structure thereof to prevent the fracture of the lubricating film, the metal surface of the bearing is excessively corroded if the addition amount of the extreme-pressure agent is too much, which causes shortening of the life of the bearing. In addition, it is necessary to take much labor of repeating a durability test for each extreme-pressure agent to determine an optimum addition amount. When a plurality of extreme-pressure agents is added to base oil, it is necessary to make a lot of experiments and investigations.
Patent document 1: Japanese Patent Application Laid-Open No. 61-12791 (scope of claims) Patent document 2: Japanese Patent Application Laid-Open No. 10-324885 (paragraph [0005])